The present invention relates to a method for detecting a starting position of each data recording block on a data recording medium such as an optical disk, and more particularly to a reliable method for detecting a starting position of each data recording block.
Heretofore in an optical disk apparatus, as shown in FIG. 4(1) of the accompanying drawings, there are provided on a recording medium chiefly ID areas in which respective track numbers, respective sector numbers and other data necessary for recording-reproducing are recorded in advance, and DATA areas in which user data are to be recorded.
At the starting position of each ID area, a pattern called "a sector mark" (SM) indicating the starting position of a sector (or data recording block) is recorded in a manner different from a data modulating manner. Each sector mark is placed between a gap (GAP) area and an ID area. In the accompanying drawings, symbol `A` designates a sector.
Since a detection signal of a sector mark is used as a timing signal for reading and writing, a very high degree of reliability is required for the detection of the sector mark. To secure such high reliability, according to conventional prior art methods, the GAP area ahead of a sector mark is detected to generate a gate signal which predicts the position of the sector mark following the GAP. The sector mark would be detected during a period of time designated by the gate signal.
From the view point as to whether or not the frequency of a recording/reproducing clock is variable, the optical-disk recording/reproducing method can be classified into a CAV (Constant Angular Velocity) method in which the frequency of an optical-disk recording/reproducing clock is unchanged over the entire surface of the disk, as shown in FIG. 4(2) of the accompanying drawings, and an MCAV (Modified CAV) method in which the recording surface of the optical disk is divided into some concentric zones and the frequency of the recording/reproducing clock in an outer zone is higher than that in an inner zone so that the recording capacity of the disk can be increased.
Also, from the view point as to how data bits `1` and `0` are recorded on the disk, the recording/reproducing method is classified into a pit-position recording/reproducing method in which `1` is represented by the position of a pit, as shown in FIG. 3(1), and a pit-edge recording/reproducing method in which `1` is represented by an edge of a pit, as shown in FIG. 3(2).
The foregoing prior art system is effective for detecting the sector mark in the pit-position recording/reproducing system regardless of which of the CAV method or the MCAV method is employed therein. However, in the pit-edge recording/reproducing system, the sector mark pattern area would be larger than that in the pit-position recording/reproducing system. In order to make the sector mark pattern different from any of the data patterns, the pits of the sector mark pattern would be elongated in the pit-edge recording/reproducing system as seen from FIG. 3, causing the following problems:
(1) The data formatting efficiency would be lowered. This is because the available storage capacity is lowered by the amount of increase of the sector mark pattern area which causes reduction of the writing area of the user's data.
(2) Automatic focusing accuracy and tracking accuracy would be lowered. This is because the sector mark pattern area in the form of elongated pits reflects a remarkably less amount of light, compared to other areas in the automatic focusing for automatically focusing a light spot on a recording surface of the disk and in the tracking control of the light spot onto grooves of the disk, thus impeding the stability of a servo system.